A lean NOx trap (LNT) catalyst or NOx occlusion catalyst (NSC) suggested in early 1990 was initially developed as a lean-burn gasoline catalyst. A separate three-way catalyst was disposed in a front end of the LNT catalyst, and due to characteristics of a gasoline engine, a catalyst temperature was higher than that of a passenger diesel engine. Therefore, alumina (Al2O3) was used as a catalyst support, and a catalyst containing a large amount (10 to 20 wt %) of high-temperature occlusion materials, such as barium (Ba), potassium (K), or the like, was used. However, at the time of using the catalyst as described above, BaAl2O4 was formed by a reaction between barium (Ba) and alumina (Al2O3), thereby causing a problem that NOx occlusion capacity of barium (Ba) may be deteriorated.
This problem was solved by introducing a cerium (Ce) component and magnesium-substituted alumina (spinel, MgAl2O4) for low-temperature occlusion. Accordingly, the LNT catalyst may be applied to an after-treatment catalyst of the passenger diesel engine, and as a catalyst support, a mixture of alumina (Al2O3) and ceria (CeO2) is used. Particularly, ceria (CeO2) plays an important role in occluding NOx at a low temperature specific to the passenger diesel engine as a material having a low catalyst temperature.
In order to prepare the LNT catalyst, 5 to 20 wt % of barium and precious metals are dispersed in alumina and ceria particles based on the entire wash coat.
At the time of preparing the spinel, an MgAl2O4 structure is formed by adding about 28 wt % of Mg (based on MgO) during an alumina synthesizing process. In general, barium spinel is prepared by impregnating barium in the spinel prepared by applying an MgO substitution ratio equal to or less than the numerical value as described above. This barium-spinel performs NOx occlusion capacity in a relatively high temperature region.
Generally, since it is difficult to prepare the LNT catalyst as a single brick catalyst due to characteristics of the LNT catalyst requiring a large amount corresponding to 70 to 90% of an engine displacement volume, a double brick or multiple brick method of disposing bricks having a predetermined diameter and length or less in series is used.
In the case of a double brick catalyst generally used in the art, a precious metal, that is, a platinum group metal (PGM: Pt, Pd, or Rh) is intensively disposed throughout the entire front-end brick (a direction toward an engine). This is to activate the catalyst immediately after starting the engine by additionally promoting heat generation by a catalytic reaction in the front-end brick firstly absorbing engine exhaust heat to promote an early raising of heating the front-end brick. However, in this case, the front-end brick may be degraded by an oxidation heat, such that NOx may be slipped (emitted) as it is.
The contents described as the related art have been provided only for assisting in the understanding for the background of the present disclosure and should not be considered as corresponding to the related art known to those skilled in the art.